Add content transfer encoding for attachments

[LienDeboosere]

add configuration for setting the content transfer encoding of an attachment

e.g:
send-message {:host "mail.isp.net"}
{:from "me@draines.com"
:to "foo@example.com"
:subject "Hi!"
:body [{:type "text/html"
:content "Test!"}
{:type :attachment
:content-type "application/octet-stream"
:content-encoding "base64"
:content (java.io.File. "/tmp/foo.txt")}
{:type :inline
:content (java.io.File. "/tmp/a.pdf")
:content-type "application/pdf"}]})

[alephyud]

Thank you! Would be great to have an option to set Content-Transfer-Encoding. @drewr Is it possible to incorporate it?